1. Field of the Invention
This invention relates to a dry method and apparatus for treating an effluent gas stream in order to facilitate the removal therefrom by conventional means of contaminating particulates particularly those in the submicron range.
2. Description of the Prior Art
In many countries throughout the world, emission standards have been or are being established to control the particulate content of effluent gases being exhausted to the atmosphere. Although these standards vary widely, most limit the particulate content of effluent gases to levels below 0.1 gr./sdcf. In many industrial applications, such as for example fiberglass furnaces, municipal incinerators, etc., such rigid standards can only be met by capturing and separating a large percentage of the submicron particulates suspended in the effluent gas stream.
Conventional dry cleaning devices lack the ability to effectively and reliably separate submicron contaminating particulate suspended in effluent gas steams. For example, in the case of baghouses where fabric filter bags are employed, experience has indicated that the submicron particulates have a tendendy to rapidly plug or "mask" the fabric interstices, thus requiring frequent disruptive bag shaking operations. Where cyclone dusts separators are employed, the submicron particulates have been found to lack the necessary mass required for efficient centrifugal separation.
Wet venturi scrubbers have also been employed for the purpose of separating contaminating particulates from effluent gas streams. Basically, a wet venturi scrubber consists of a constriction in the conduit carrying the contaminated effluent gas stream. The effluent gas stream is accelerated through the venturi constriction, and a liquid (usually water) is injected into the gas stream at the venturi throat. The high gas velocity atmoizes the liquid and the relative velocities between the contaminating particulates and the liquid droplets result in a combination of one with the other through inertial impaction. The liquid droplet and their captured contaminating particulates are then separated from the effluent gas. While this technique can lead to higher collection efficiencies, this advantage is offset to a considerable extent by other associated problems.
For example, it is known that the efficiency of the inertial impaction technique can be improved by reducing the size of the target liquid droplets. This however requires higher gas velocities with accompanying pressure drops across the venturi of 30"-60" w.g. A pressure drop of 30" w.g. results in an excessively high energy usage of 240 kWh per million cu. ft. of gas cleaned. Attempts at reducing the pressure drop across the venturi have not been successful, primarily because a high gas velocity is essential at the venturi throat in order to achieve optimum atomization of the injected liquid and still have a remaining differential velocity between the liquid droplets and the contaminating particulates which is sufficient to produce the desired inertial impaction. Some thought has geen given to injecting a pre-atomized liquid spray into the gas stream in order to accommodate reduced gas velocities through and reduced pressure drops across the venturi, but any advantage gained in this regard has been found to be offset by the power required to pre-atomize the liquid.
Another problem with wet venturi scrubbers is that the atomized liquid droplets combine with acid components of the effluent gas stream to produce a high corrosive medium. This in turn makes it necessary to employ ducts and associated downstream equipment constructed of expensive exotic corrosion resistant materials. Even when this is done, however, corrosion related maintenance problems are encountered. Moreover, the resulting acid solutions must be neutralized, and even after this is done, disposal problems are encountered.
Other known gas cleaning arrangements have involved the injection of solid material into the effluent gas stream. An example of one such arrangement is shown in U.S. Pat. No. 2,875,844. Such arrangements have resulted in little or no capture of submicron particulates because the solids have been dumped into the effluent gas stream at the outside diameter of the conveying duct as a dense agglomerate. By the time dispersion occurs, a condition which is essential for efficient capture, the solids have attained approximately the same velocity as that of the effluent gas and the contaminating particulates suspended therein. Without an adequate relative velocity between the contaminating particulates and the dispersed target particulates, effective particulate capture through inertion impaction is an impossibility.
According to U.S. Pat. Nos. 3,969,482 and 3,995,005, solid particulate material for sorbing and/or reacting with acid gases may be dispersed in a secondary air stream and radially injected through one or more points along an effluent gas carrying conduit. However, in the absence of adequate relative velocity between the gas streams together with means for initially distributing the secondary stream into the effluent stream, little or no capture of contaminating particulates occurs, especially at conduit positions radially removed from the injection points. By the time the added material is dispersed throughout the effluent gas, the relative velocity of the streams approaches zero.
While electrostatic precipitators have met with some success, their operation has been plagued by corrosion, buildups of oils and fats where combustion practices are less than optimum, and variations in the conductivity of the contaminating particulates.